1. Field of the Invention
The present invention relates to a gateway for connecting a network where data propagation delays vary with another network and a data transfer method suitable for the gateway.
2. Related Art
Gateways are the indispensable components in order to achieve multimedia communications between terminals connected to heterogeneous networks that use different protocols and have different network characteristics. This type of gateway receives data following the protocol for the network to which the source terminal belongs and transmits the data pursuant to the protocol used in the network to which the destination terminal belongs. Consequently, the data from the source terminal passes through the network to which the source terminal belongs, the gateway, and the network to which the destination terminal belongs.
For multimedia communications, terminals complying with ITU-T Recommendation H.323 are commonly employed on packet-switched networks, such as the Internet, whereas terminals complying with ITU-T Recommendation H.324 are employed on circuit-switched networks, such as PSTN (public switched telephone network). Since Recommendations H.323 and H.324 use different transport protocols: RTP/UDP for H.323 and multiplexing protocol prescribed in H.223 for H.324, it is necessary to carry out the protocol conversion in a gateway for interconnection. Additionally, it is required to compensate for the difference in the network characteristics; delay, delay jitter and error characteristics, to achieve high quality multimedia communications.
Data propagation delays occur in networks due to various reasons, and delay times for data units may be different from each other. This kind of unwanted variation in the propagation delay time is called delay jitter. The causes of delay jitter may, for example, be the store-and-forward type exchange in a packet-switched network, packet or frame queuing at a node on the transmission path, or the fact that data units arrive at the gateway through different paths.
Even in a circuit-switched network, delay jitter is caused by simultaneous transmission of different forms of data. For example, multimedia terminals like videophones multiplex video, audio and/or application data into one bit stream. In such a case, while there will be data units that are immediately transmitted, there will also be data units that are kept waiting until the currently transmitted data unit is completed. This type of delay jitter could occur at both terminals and gateways, and would be larger as transmission bit rate becomes lower.
When delay jitter occurs, time intervals of data units in the source terminal cannot be maintained in the destination terminal. For transmitting data units, such as audio data units, for which highly precise synchronization is required, it is preferable to dejitterize for data units.
As means for dejitterizing, some communicating terminal can absorb a certain degree of delay jitter when it receives data units. For example, a communicating terminal temporarily stores data units, which are received with various delays, and outputs them successively after a series of data units have accumulated. However, since the occurrence factors and characteristics of delay jitter differ according to the network, it is difficult to know in advance how large the jitter among delays of data units which can be received by a destination terminal is. Therefore, the size of delay jitter which is permitted (or absorbed) in each destination terminal is often based on the likely maximum delay jitter that may occur in propagation through the network to which the terminal belongs. This results in the increase of end-to-end delay, which would be serious problem for real time communication.